Due to imperfections and process variations, machinery used to manufacture semiconductor devices have offset times with respect to calculated processing times. A calculated processing time is a theoretical time for completing a processing step assuming ideal conditions. An offset time is a variation between the calculated processing time and an actual time for completing the processing step. In some approaches, the offset time is determined by processing a group of test wafers and analyzing the test wafers to determine whether the processing step was properly completed, whether the processing step was not properly completed or whether the test wafers were over processed.
Preventive maintenance is performed on machinery used to fabricate semiconductor devices in order to maintain the machinery in working order. During preventive maintenance, a backup machine is used in place of the machinery. An offset time of the backup machine is often different from an offset time of the machinery normally used to fabricate the semiconductor devices.